powerbands

[matchuk]

does anyone know in detail how powerbands work, ive searched online a bit and cant really find the info im looking for...

i was just thinking how come acceleration isnt constant throughout the rpms towards redline, in theory it should be ...

also why are race engines designed for a narrower powerband isnt the larger the powerband the better

[TheSaint]

cam lobe?

a lumpy cam lobe sux down low but is really good up high, wer is a small cam lobe is great down low but has no top end at all

VTEC eliminates this by having both

[sever_all_ties]

Race engines usually have a powerband suited to where the RPMs are at most of the time. So if its spent most of the time between 6-9k RPM then thats where the powerband will be tuned for, as having power anywhere else would be a waste of time.

[aaronng]

i was just thinking how come acceleration isnt constant throughout the rpms towards redline, in theory it should be ...
Does your theory assume constant torque throughout the RPM range? There are cars tuned like that and do have constant acceleration from low RPM. But in reality, it isn't for smaller engines because we keep demanding more and more power from it by using aggresive cams.. Because the cam lobes used to drive the valves have a particular range of RPM where they operate at optimum, you get non-linear acceleration with aggressively tuned engines like a b16a. Oh, with VTEC, you get 2 cam profiles, so you are able to widen the powerband RPM range.



also why are race engines designed for a narrower powerband isnt the larger the powerband the better
The more aggressive the cam profile, the higher the RPM at which peak torque occurs. The max RPM is constrained by how the engine is designed and the parts used. Of couse, you can use pneumatic valves to eliminate cams and get a wide powerband, like how F1 cars have a powerband from about 13000 to 19000 RPM. That's a 6000rpm powerband, which is as wide as the entire RPM range of our normal cars.

[matchuk]

hmmm im prob not clear on what im trying to understand,

im aware of aggressive cams but what i dont understand is why does some RPM range "Actually" produce stronger acceleration opposed to others i think i read from wiki or something its something to do with engine temperature oil pressure or something...

[aaronng]

hmmm im prob not clear on what im trying to understand,

im aware of aggressive cams but what i dont understand is why does some RPM range "Actually" produce stronger acceleration opposed to others i think i read from wiki or something its something to do with engine temperature oil pressure or something...

RPM range doesn't produce stronger acceleration. It's the way the engine is tuned through the combination of cams, ECU maps and internals. You can have a car with strong acceleration at low RPM but weak acceleration at high RPM (old gen pushrod engines) or weak acceleration at low RPM but strong at high RPM (b16a, b18c, f20c)

[JohnL]

hmmm im prob not clear on what im trying to understand,

im aware of aggressive cams but what i dont understand is why does some RPM range "Actually" produce stronger acceleration opposed to others i think i read from wiki or something its something to do with engine temperature oil pressure or something...

Matchuk,
An engines ability to produce power is directly related to how much air it can draw in per revolution of the crankshaft (fuel is then injected in a quantity to account for this volume of air, but there's no point injecting more fuel if less air is drawn in). This is called 'volumetric efficiency', and if an engine can draw in enough air into each cylinder that the air pressure inside that cylnder (before the compression stroke begins) is equal to atmospheric pressure then that engine has 100% volumetric efficiency.

However, because air doesn't weigh nothing (i.e. has mass) it's hard to get the air moving into the engine fast enough to achieve 100% VE considering how little time there is for air to be drawn into the cylinder.

Many higher performance naturally aspirated engines (non supercharged) can reach 100% VE (and a bit more), but only only over a relatively narrow rpm range. This is acheived with various factors involved with camshaft timing (valves opening and closing at particular moments and periods of time), cylinder head design, and the 'tuned' length / size of induction and exhaust manifolds and pipes etc. But, these 'tricks' only work well within certain rpm parameters, the more aggressive the 'tricks' the narrower the working rpm range will be, but the more power is likely to be produced in that rpm range because the engine is drawing in a lot more air in that range.

All these 'tricks' have to do with getting the air flow into the engine to happen very quickly, and when working very well (within the power band) an engine can reach 100% VE (sometimes more), but outside the rpm of the 'powerband' (both below and above) the VE will fall to below 100%, often very substantially.

Supercharged engines (and turbos are a kind of supercharger) can also suffer from low VE in certain parts of the rpm range as with a naturally aspirated engine, but with supercharging it's 'easier' to fill the cylinders to 100% VE (and usually considerably more) because the supercharger is forcing air into the engine at well over atmospheric pressure. As a result a superchareged engine can typically produce more power (often a very great deal more) over a wider rpm range than a naturally aspirated engine.

However, superchargers present other difficulties and trade offs relative to naturally aspirated engines (not the least being added complexity and stress on the engine), meaning they are not always a practical proposition. I said above that it was 'easier' to fill the cylinders with a supercharger, but that only true in principle, in practice it's not so easy due to complexity, stress, and cost.

I hope this is the sort of answer you were after, but bear in mind that what I've said is a rather simplistic and basic explanation, this is in reality all rather complex.

[matchuk]

Matchuk,
An engines ability to produce power is directly related to how much air it can draw in per revolution of the crankshaft (fuel is then injected in a quantity to account for this volume of air, but there's no point injecting more fuel if less air is drawn in). This is called 'volumetric efficiency', and if an engine can draw in enough air into each cylinder that the air pressure inside that cylnder (before the compression stroke begins) is equal to atmospheric pressure then that engine has 100% volumetric efficiency.

However, because air doesn't weigh nothing (i.e. has mass) it's hard to get the air moving into the engine fast enough to achieve 100% VE considering how little time there is for air to be drawn into the cylinder.

Many higher performance naturally aspirated engines (non supercharged) can reach 100% VE (and a bit more), but only only over a relatively narrow rpm range. This is acheived with various factors involved with camshaft timing (valves opening and closing at particular moments and periods of time), cylinder head design, and the 'tuned' length / size of induction and exhaust manifolds and pipes etc. But, these 'tricks' only work well within certain rpm parameters, the more aggressive the 'tricks' the narrower the working rpm range will be, but the more power is likely to be produced in that rpm range because the engine is drawing in a lot more air in that range.

All these 'tricks' have to do with getting the air flow into the engine to happen very quickly, and when working very well (within the power band) an engine can reach 100% VE (sometimes more), but outside the rpm of the 'powerband' (both below and above) the VE will fall to below 100%, often very substantially.

Supercharged engines (and turbos are a kind of supercharger) can also suffer from low VE in certain parts of the rpm range as with a naturally aspirated engine, but with supercharging it's 'easier' to fill the cylinders to 100% VE (and usually considerably more) because the supercharger is forcing air into the engine at well over atmospheric pressure. As a result a superchareged engine can typically produce more power (often a very great deal more) over a wider rpm range than a naturally aspirated engine.

However, superchargers present other difficulties and trade offs relative to naturally aspirated engines (not the least being added complexity and stress on the engine), meaning they are not always a practical proposition. I said above that it was 'easier' to fill the cylinders with a supercharger, but that only true in principle, in practice it's not so easy due to complexity, stress, and cost.

I hope this is the sort of answer you were after, but bear in mind that what I've said is a rather simplistic and basic explanation, this is in reality all rather complex.


<--- absolute legend

so fuel is actually injected after judging the amount of air pressure within the cylinders

so in theory DOHC = more time for air to get in or allows more = alot stronger power than sohc across any rpm range

[JohnL]

<--- absolute legend

Only at lunchtime...


so fuel is actually injected after judging the amount of air pressure within the cylinders

Not really. The amount of fuel to be injected at any moment in varied conditions is determined by thorough testing of prototype engines on dynamometers (a machine that measures engine torque), among other things.

This information is included in the ECU programming, which measures various engine parameters with various sensors, corelates this with the information stored in it's hard memory and adjusts the fuel injection (and ignition timing) to suit. The programming covers a large number of variables, and is rather complex (at least it is for me).

The ECU has some latitiude to make ad hoc adjustments according to measurements of oxygen content in the exhaust gas in relation to other parameters, though this is typically only in partial throttle running with a motor at operating temperature (at full throttle and in cold running the ECU will typically run the injection and timing from 'hard maps' stored in the memory with no input from the oxygen sensor).


so in theory DOHC = more time for air to get in or allows more = alot stronger power than sohc across any rpm range

There's no difference between SOHC and DOHC engines in the time needed for air to be drawn into the cylinder, this is simply governed by the rpm (i.e. revolutions per minute), and the amount of time the valves are open (to put it very simplistically). As I said before, this is a very complex area, and I simply can't give you all the info you need to make proper sense of it all here. You need to find some books on engine theory and have a good read.

The difference between SOHC and DOHC cam engines is mostly to do with the mass of the valve gear (valve 'train'), and DOHC designs allow more freedom in arranging the angles of the inlet and exhaust valves relative to each other, which in turn allows more freedom in the design of the combustion chamber shape for greatest efficiency, whilst also keeping valve train mass low.

With valve trains you want the least possible mass, and this generally means the least number of components per valve. You want the least possible valve train mass because if the valve train is heavy you need very stiff valve springs to prevent the valves 'floating' at high rpm ('floating' meaning that when the valve reaches maximum valve lift it continues to lift even after the cam has stopped lifting it, due to the inertia of the mass in the valve train), and this can damage the valve train and can adversely affect power output.

This is a particular problem for high rpm engines, and with heavy valve trains you need much stiffer springs to resist float at higher rpm, but stiffer springs can cause greater possibilty of valve train damage at higher rpm and has implications for things such as cam lobe wear etc.

The valve train components in question include the valve itself, the valve spring, the valve cap and collets, and the valve rocker (for pushrod engines we can also include the pushrod and the 'cam follower', both heavy in valve train terms).

If you have a SOHC you can design it without rockers, but this limits you to having all the valves in a straight line along the top of the motor directly under the camshaft (which also means you can only have two valves per cylinder because there won't be room for four valves in a staight line across the top of each cylinder).

With SOHC if you want to have four valves per cylinder or to place a single inlet and single exhaust valve to each side of the cylinder and angle them inwards toward the combustion chamber (which is good for airflow into and out of the motor and also allows a more efficient combustion chamber shape), then you need the rockers to operate the valves indirectly from the centrally located single camshaft (more mass).

With DOHC engines you can operate the valves directly from each cam without the need for rockers (less mass), and you can place the valves at any angle you want for greatest efficiency. 'Efficiency' includes the design of the ports into and out of the head to allow a less restrictive path (less tight bends) for gas flow, and also a combustion chamber shape (i.e. the shape of the 'depression' in the cylinder head) that allows a more efficient 'burn' of the fuel / air mixture.

[NightKids]

Is this guy a professor or something? So basically what your telling me is .... VTEC >> VVTi-L >> Mivec

[TECBOY]

^lol......

[JohnL]

Is this guy a professor or something? So basically what your telling me is .... VTEC >> VVTi-L >> Mivec

I'm only talking about the differences between SOHC and DOHC engines in general. When you start adding various different types of variable valve timing arrangements you're talking about something else.

I'm not really familiar with all the different types of variable valve timing mechanisms, I know VTEC and VVTi in principle, but wouldn't know a Mivec if it bit me (and I'm only assuming it's a variable valve timing thingemabob doohickey whatsit).

[ricki_kalsi]

A powerband is defined as the area / region between Peak Torque (NM), and Peak Power (kW).

The reason why people constantly rev above their peak Power RPM point - I have no idea. They think it makes more power, but as you see on Dyno graphs, it's rubbish.

[ZeForce]

The reason why people constantly rev above their peak Power RPM point - I have no idea. They think it makes more power, but as you see on Dyno graphs, it's rubbish.

You rev past your peak power so you land higher in the rev range in the next gear. Usually power wont drop off straight away anyways and you really want to stay in the lower gears for as long as possible since they multiply your torque more.

[ricki_kalsi]

If you change gears at 7800RPM where peak Power is... I can gauretee you will drop in after your Peak torque, in a DC2R anyways.

[Feverpitched]

Powerbands, isn't that what Pat Cash used to wear?

[ZeForce]

If you change gears at 7800RPM where peak Power is... I can gauretee you will drop in after your Peak torque, in a DC2R anyways.

lol.... that still doesnt change the fact your car would accelerate faster if you shifted at 8500rpm

[ricki_kalsi]

lol.... that still doesnt change the fact your car would accelerate faster if you shifted at 8500rpm

But it doesn't... you in fact loose power, when you should be using the next gear to be getting up to the high rev in the power band.

[ZeForce]

But it doesn't... you in fact loose power, when you should be using the next gear to be getting up to the high rev in the power band.

Sorry but your wrong :thumbdwn:

[caruso_19]

ye mate.....im going to have to agree with ZeForce on this

[fatboyz39]

lol.... that still doesnt change the fact your car would accelerate faster if you shifted at 8500rpm

we should jsut test this theory out...hahahaha... would be such a waste of time tho...

stock dc2r shifting @ 7800rpm and another shifting @ 8500rpm see who wins.

[aaronng]

Time to organise a WSID meet? ;)

[Elwood]

A powerband is defined as the area / region between Peak Torque (NM), and Peak Power (kW).

Or more simply..

How long the engine is producing its peak torque.


we should jsut test this theory out...hahahaha... would be such a waste of time tho...

stock dc2r shifting @ 7800rpm and another shifting @ 8500rpm see who wins.

Too many variables. :P

[JohnL]

My thoughts on this (just MO):

Peak power rarely coincides with the redline, peak power being nearly always below redline to varying degrees. Redline is typically a limit set by the manufacturer as an rpm to which it's safe to rev the motor without significant risk of damage, it's not intended to be the point at which the next gear should be selected for maximum acceleration. As far as acceleration is concerned, the redline is an arbitrary and irrelevant limit.

On a given motor, the most advantageous rpm at which to shift to the next higher gear depends on the breadth of the power band, the size of the 'gaps' between the gear ratios, and just how quickly power drops away after peak power (and before peak torque).

If power falls flat on it's face just after PP then there's no point in revving past PP by more than a small amount (if the redline is much beyond PP and PP is falling fast it'll take too long to achieve redline after PP has been passed).

If the gaps between ratios are too wide for the power characteristics of the engine then this means that when the next gear is selected the rpm may fall significantly below PT. In this case it would be more worthwhile revving past PP so that when you select the next higher gear you are at or at least nearer PT.

On the other hand, depending on the gear ratios and powerband breadth, if you rev to the redline, when the next gear is selected the rpm may be significantly above PT, so you may be 'losing' some of the useful powerband, which is a double waste if you need to rev past PP to reach the redline.

This all depends on how much power is being lost above PP to redline, and how far below PT the rpm would be after you shift. It's a compromise either way, and you need to find the least disadvantageous compromise for each individual case. In some cases it will be best to shift at redline, in others at PP, and this may even be different with the same car depending on which gear is next being selected (the gaps between ratios aren't necessarily evenly spaced).

The ideal is for the gear ratios to complement the powerband of the engine, so that each time you shift up at PP the rpm drop exactly to PT, then again with the next shift etc. This is the main reason why high performance engines typically require close ratio gearboxes (close ratio boxes also tend to allow faster shifts). Most cars in stock trim will have a fairly wide powerband that matches the fairly wide gear ratios fairly well, and be relatively forgiving of being a bit over or under revved when shifting gear.

Too many people 'tune' the bejeesus out of their engine and end up with a peaky powerband that poorly matches the ratios in the gearbox. The car may feel fast and exiting when in the powerband, but in the bigger picture it will be slower than if it had a bit less power with a flatter / less peaky powerband, or better, if appropriate ratios were installed.

My CB7 redlines at 6300rpm, but at about 5500 I can feel the power start to drop away (the trials of not having VTEC!), so I shift at 5500. I find this results in a stronger initial surge of acceleration in the next gear than if I shift at 6300 (though admittedly this utterly subjective, and may possibly be attributable to feeling the power dropping away at higher rpm in the lower gear).

This implies that shifting at 5500 in my particular car places the rpm closer to peak torque in the next gear than it does when shifting at 6300, i.e. at the rpm you get in the next gear after shifting at 6300, even if power may be increasing torque is decreasing, so it's better to shift at 5500 so you don't 'waste' PT.

[Elwood]

^Good write up.

I can't feel my dc2r dropping power after PP though, maybe im just having too much fun vtakking ;)